Sudden rash on toddler.

Ka

Hi all, while giving my 16 month old a bath tonight I noticed a bunch of small raised bumps on his back and belly. Nowhere else. They aren't bothering him at all and they weren't there this morning. He has never had skin issues and I haven't changed detergent or anything lately. Any idea what would cause a sudden rash like this? It's not on his face/hands/feet.